The present invention relates to an improvement of a microtron electron accelerator and more particularly to an improvement in structure of an electron source and an accelerating cavity small in size and suitable for obtaining a high-energy electron beam stably and to optimization of electron accelerating conditions.
The microtron electron accelerator is an apparatus for accelerating electrons with a microwave. A microtron electron accelerator of a conventional structure is formed of an electromagnet 2 for generating a uniform magnetic field B and an accelerating cavity 1 accepting microwave electric power 3 for generating high-frequency accelerating electric field E as shown in FIG. 7. On the inner wall surface of the accelerating cavity 1, there is provided a hot cathode 4. Electrons e are emitted from the hot cathode 4 and accelerated by the high-frequency accelerating electric field E within the accelerating cavity 1. The accelerated electrons e are deflected by the uniform magnetic field B and ejected from the accelerating cavity 1 through a hole 63 allowing electron beam to pass through (hereinafter briefly called "electron beam through-hole") formed in the wall of the accelerating cavity 1. The ejected electrons e draw a circular trajectory 91 in the uniform magnetic field B and are injected into the accelerating cavity 1 through an electron beam through-hole 61. Here, the electrons e are further accelerated by the high-frequency accelerating electric field E and ejected from the accelerating cavity 1 through the electron beam through-hole 63, and, then, they draw a still larger circular trajectory 92 in the uniform magnetic field B and are again injected into the accelerating cavity 1 through the electron beam through-hole 61. Such operations are repeated and, thereby, the electrons e are progressively accelerated to obtain higher energy and trace successively greater trajectories 93, 94, and 95, and trace a final circular trajectory 96 and are extracted from the magnetic field B as electrons with desired energy through an extracting pipe 8 provided in the final circular trajectory 96. Since the amount of the output current flow of the electron beam finally extracted from the apparatus is small with the apparatus of the structure shown in FIG. 7, a proposal to increase the amount of the output current flow by obliquely forming the surface on which the cathode 4 is provided in the accelerating cavity 1, so that the effective cathode area is increased, is disclosed in the gazette of Japanese Patent publication No. Hei 1-31680.
In the above described apparatus of a conventional structure, since the cathode 4 is provided on the inner wall surface of the accelerating cavity 1, the material for cathode evaporated from the heated cathode by heating the cathode 4 was liable to adhere to the inner wall surface of the accelerating cavity 1. Thus, the inner wall surface of the accelerating cavity 1 was contaminated by the adhesion of the evaporated cathode material to it, and because of this, there were caused such problems that the Q-value of the accelerating cavity 1 was decreased making it difficult to satisfactorily accelerate the electrons or discharges were produced due to bad resistivity for voltage. Therefore, it was liable to occur in the apparatus of conventional structure that, while the electron beam accelerating characteristic in the accelerating cavity 1 is satisfactory in the early stage of its use, the electron beam accelerating characteristic in the accelerating cavity 1 becomes gradually deteriorated by aged deterioration due to the above described adhesion of cathode material to it. Thus there has been a problem that an electron beam with a desired large amount of current flow is not obtainable stably.